A resonator is an electronic device used for setting up and maintaining an oscillating electrical signal of a given frequency. Conventional resonators typically include electronic circuitry in combination with a mechanical oscillator element (e.g., a quartz crystal, a ceramic element or a resonance circuit). Resonators are used in many electronic devices, such wireless radio frequency (RF) equipment, for generating outgoing signals of a particular frequency, and filtering incoming signals.
In most electronic devices that require signal generation and filtering, conventional resonators are used. Such resonators have a high Q-factor (i.e., a sharp resonance peak) good frequency stability and are generally very reliable. However, conventional resonators tend to be relatively large (i.e., on the order of 1 cm), so that alternatives are preferred when trying to fabricate a compact electronic device.
One alternative to conventional crystal-based resonators is a microelectromechanical systems (MEMS) resonator. Generally, a MEMS device is a microdevice that integrates mechanical and electrical elements on a common substrate using microfabrication technology. The electrical elements are typically formed using known integrated circuit fabrication technology. The mechanical elements are typically fabricated using lithographic and other related processes to perform micromachining, wherein portions of a substrate (e.g., silicon wafer) are selectively etched away or added to with new materials and structural layers.
FIG. 1 shows one type of prior art MEMS resonator 10 formed on a substrate 12. MEMS resonator 10 has a cantilever-type beam 16 arranged between a lower electrode 20 and an upper electrode 26. Beam 16 is electrostatically driven by the upper and lower electrodes to oscillate at a given frequency. FIG. 2 shows another type of prior art MEMS resonator 40 similar to MEMS resonator 10 but having a bridge-type beam 46 and an optional bridge-type upper electrode 50. Beam 46 is anchored to the substrate at its ends so that the center portion of the beam can be driven to oscillate by being electrostatically deflected between the upper and lower electrodes. FIG. 3 shows yet another prior art MEMS resonator 70 called a “breathing bar resonator.” MEMS resonator 70 includes a bar-type beam 76 fixed to substrate 12 with a single central support member 80. Side electrodes 84 and 86 are located on either side of beam 76 with small gaps 88 in between. Electrodes 84 and 86 drive beam 76 to expand and contract (i.e., resonate) along its long axis in a manner that resembles breathing.
MEMS resonators are desirable for many miniaturized electronic devices because they can be made smaller than conventional resonators by an order of magnitude or more. However, because a MEMS resonator relies on the mechanical oscillation of a very small (i.e., micron-sized) beam as opposed to the vibration of a relatively large oscillation element (e.g., a centimeter-size crystal), the resonator must be packaged in a vacuum to eliminate air damping of the beam's oscillation. Vacuum packaging is also necessary to avoid the adsorption of contaminants, which can alter the resonant frequency of the beam.
A challenge in fabricating MEMS resonators is the vacuum packaging step. Various techniques for vacuum packaging a MEMS resonator are available, such as wafer bonding, flip-chip, and thick membrane transfer techniques. However, these techniques require dedicated alignment/bonding technologies that are relatively complicated to apply to MEMS packaging. Another technique for MEMS vacuum packaging involves using a permeable polysilicon release process. While conceptually simple, such a process has proven very difficult to control and has yet to lead to a manufacturable MEMS resonator vacuum packaging process.
In the Figures, like reference numbers refer to like elements.